Polyimide resins obtained by polymerizing a tetracarboxylic dihydride and a diamine exhibit excellent heat resistance, chemical resistance, mechanical strength, and electric characteristics and, therefore, have been used for various purposes. However, in general, aromatic polyimides have structures including long conjugated systems composed of aromatic rings and imide groups on a consecutive basis and, thereby, the light in a visible region is absorbed, so that the resins are colored with yellow to brown. Consequently, there are limitations on the use for purposes in which achromatic transparency is particularly required, for example, optical material system purposes.
As for resins for optical materials, polymethyl methacrylates and polycarbonates have been used previously. They exhibit excellent transparency, but each exhibits low heat resistance. Therefore, it is difficult to apply them to the purpose of using at high temperatures. Furthermore, polycarbonates exhibit heat resistance slightly higher than the heat resistance of the polymethyl methacrylates, but exhibit high birefringence. Therefore, there is a problem in application to high-precision optical elements.
Polyimide resins have also been used for printed wiring boards, interlayer insulating films, and the like previously. The polyimide resins which have been used for these purposes are aromatic. In typical cases, the dielectric constants of these aromatic polyimides are assumed to be 3.0 to 4.0 (refer to Non-Patent Document 1 as described below).
In recent years, regarding the development of the large scale integrated circuit (LSI), wirings have become finer because of increases in operation speed, and still higher insulating property is required of interlayer insulating films to be used for these purposes. However, known polyimide resins cannot be applied because of high dielectric constants.
Consequently, research has been conducted on reduction in dielectric constant while the heat resistance of the polyimide resin is maintained. One of which is introduction of an atom having a small molar polarizability. Typically, it is a method in which a fluorine atom is introduced (refer to Non-Patent Document 2 as described below). However, industrial production of a polyimide resin containing a fluorine atom has problems in availability of raw materials and cost.
Research has also been conducted on reduction in dielectric constant by forming fine holes in a polyimide resin. However, the formation of such a structure requires a production step of dispersing templates formed from a heat-decomposable material into a polyimide resin homogeneously and thermally decomposing the templates after film formation. Therefore, there is an essential problem in that the production process becomes complicated (refer to Non-Patent Document 3 as described below).
In general, the polyimide resin has a low degree of solubility in a solvent. Consequently, a precursor, in the state of polyamic acid, of the polyimide resin is usually applied as a solution and is converted to the polyimide by a heat treatment at high temperatures. Therefore, there are workability limitations. In particular, there are problems in that, for example, the polyamide cannot be used in the case where a portion, at which the polyimide is intended to be disposed, tends to be irreversibly damaged by heat. Usually, shrinking occurs during cooling after the high temperature treatment and in many cases, serious problems, e.g., peeling and cracking of a film, occur because of a thermal stress originating therefrom.
Under these circumstances, some proposals have been made regarding achromatic, transparent, low-dielectric, solvent-soluble polyimide resins while the heat resistance is maintained. In one of the methods thereof, a polyamide resin is produced by using a tetracarboxylic dihydride not including an aromatic ring, but including an aliphatic group or a diamine. For example, a polyamide resin derived from a tetracarboxylic dihydride having a structure in which alicyclic ring skeletons are condensed on a consecutive basis has been proposed (Patent Document 1 as described below). However, the birefringence of this polyimide resin is not always sufficient. Furthermore, synthesis of the raw material is multistage and complicated and, therefore, industrial production has a problem.
A polyamide resin derived from 1,2,4,5-cyclohexanetetracarboxylic dihydride has been proposed as a substrate material for a thin film transistor (Patent Document 2 as described below). However, according to the examples described therein, the resulting polyimide resin has high transparency, but is colored with light brown. Therefore, it cannot be used for purposes in which high achromatic transparency is required.
In general, introduction of the aliphatic group contributes to improve the transparency and reduce the dielectric property. On the other hand, there are problems in that the heat resistance is reduced and high thermal stability which is a feature intrinsic to the polyimide resin is lost. Furthermore, the imide group itself has high water absorption property, and the water absorption property of general polyimide resins are assumed to be 3 percent by weight or more. This high hygroscopic property is based on the imide group and, therefore, cannot be overcome by merely introducing the aliphatic group.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-96070
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-168800
Non-Patent Document 1: 54th (2005) Symposium on Macromolecules Preprints 2Pc095
Non-Patent Document 2: Macromolecules, 27, 5964 (1994).
Non-Patent Document 3: Polymer, 36, 2685 (1995).